A large static magnetic field is used by Magnetic Resonance Imaging (MRI) scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This large static magnetic field is referred to as the B0 field.
During an MRI scan, Radio Frequency (RF) pulses generated by a transmitter coil cause perturbations to the local magnetic field, and RF signals emitted by the nuclear spins are detected by a receiver coil. These RF signals are used to construct the MRI images. These coils can also be referred to as antennas. Further, the transmitter and receiver coils can also be integrated into a single transceiver coil that performs both functions. It is understood that the use of the term transceiver coil also refers to systems where separate transmitter and receiver coils are used. The transmitted RF field is referred to as the B1 field.
MRI scanners are able to construct images of either slices or volumes. A slice is a thin volume that is only one voxel thick. A voxel is a small volume over which the MRI signal is averaged, and represents the resolution of the MRI image. A voxel may also be referred to as a pixel herein.
Dixon methods of magnetic resonance imaging include a family of techniques for producing separate water and lipid (fat) images. The various Dixon techniques such as, but not limited to, two-point Dixon Method, three-point Dixon method, four-point Dixon method, and six-point Dixon Method are collectively referred to herein as Dixon techniques or methods. The terminology to describe the Dixon technique is well known and has been the subject of many review articles and is present in standard texts on Magnetic Resonance Imaging. For example “Handbook of MRI Pulse Sequences” by Bernstein et. al., published by Elsevier Academic Press in 2004 contains a review of some Dixon techniques on pages 857 to 887.
The journal article Xiang, “Two-Point Water-Fat Imaging With Partially-Opposed-Phase (POP) Acquisition: An Asymmetric Dixon Method,” Magnetic Resonance in Medicine, 56:572-584 (2006) discloses a Dixon method where water and fat magnetization vectors are sampled at partially-opposed-phase instead of being exactly antiparallel.
The journal article Berglund et al., “Two-point Dixon Method With Flexible Echo Times,” Magnetic Resonance in Medicine, 65:994-1004 (2011) discloses a Dixon method where a water-only and a fat-only image are produced from a dual-echo acquisition.” The US patent application US2011/0254547 concerns a method of magnetic resonance imaging that involves water-fat separation. A weighting map that represents the likelihood that one chemical species is depicted as another (water-fat swap) and a weighting map that characterises the smoothness of field map variations are utilised to achieve a robust water-fat separation.